Piezoelectric devices, such as piezoelectric benders or actuators, generally consist of a piezoelectric material that deforms when an electric field is applied across it. Additional materials may be bonded with the piezoelectric material, such as metallic layers that act as electrodes, insulating materials to prevent current from flowing between particular areas of the device, and adhesives to bond the various layers together.
A typical piezoelectric device, such as a piezoelectric actuator, may be comprised of an electroactive layer, comprised of various types of ceramics, disposed between a reinforcing layer and a top or conductive layer, although other configurations, such as electroplating, are also possible.
It is also generally known to provide a pre-stress to the piezoelectric material, thus increasing the displacement capability. A method for producing pre-stressed piezoelectric devices typically includes the steps of creating a composite structure by layering the ceramic layer with one or more supporting layers with thermoplastic adhesive layers therebetween. The composite structure is heated to a temperature above the melting point of the adhesive and below the Curie temperature of the ceramic (in order not to destroy the piezoelectric characteristics of the ceramic). The composite structure is then allowed to cool to an ambient temperature during which time the adhesive sets and bonds the layers. Due to the differences in the coefficients of thermal expansion of the ceramic layer, the supporting layer(s), and the adhesive, the ceramic layer becomes typically compressively stressed as the composite structure cools. In addition, due to the greater thermal contraction of at least one of the supporting layers than the ceramic layer, the structure, as it cools, deforms into a domed, or arcuate, shape.
It has also been recognized that the amount of pre-stress imparted to the ceramic layer is dependent on the changes in temperature of both the ceramic and supporting layers during the cooling step of the process. As seen in U.S. Pat. No. 6,156,145 issued to Clark, a method of manufacturing pre-stressed piezoelectric devices is disclosed, the method allowing for the supporting layer to have a greater temperature drop during the cooling step than the temperature drop of the ceramic layer. In this method, the supporting layer has an adhesive applied, and then the supporting layer is heated to above the melting point of the adhesive. A ceramic layer is positioned onto the heated supporting layer, with the melted adhesive disposed therebetween; the ceramic layers is preferably initially at a lower temperature than the supporting layer. Pressure is applied to the structure, and the structure is allowed to cool to an ambient temperature. Thus, the temperature drop to ambient temperature of the supporting layer is greater than that of the ceramic layer. And due, in part, to the differences in temperature drop, the ceramic layers is compressively pre-stressed as the structure cools. However, this method requires that the ceramic layer be applied once the adhesive is melted, which may be difficult. In addition, to vary the amount of pre-stress imparted to the ceramic layer, the difference between the individual temperatures of both the supporting layer and the ceramic layer may need to be adjusted.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.